front matter 1

Cellular roof plate

Fibrous roof plate

Floor plateFloor plate

Expanding

dorsal

canal

Expanding

dorsal

canal

Central

canal

Receding

ventral canal

Receding

ventral canal

Ventral horn

motoneurons

are beginning

to segregate into

motor columns

Ventral horn

motoneurons

are beginning

to segregate into

motor columns

Migrating

ventral horn

interneurons

Migrating

ventral horn

interneurons

Intermediate

interneurons

Intermediate

interneurons

Roof plate

Floor plate

Dorsal root ganglion

Dorsal root

Ventral root

Spinal nerve

Expanding

gray matter

Expanding white matter

Lateral plate

(neuroepithelium, NEP cells)

Dorsal root

bifurcation zone

Expanding tip

Active ventral

interneuron source?

Active ventral

interneuron source?

Depleted motoneuron

source?

Depleted motoneuron

source?

Active source of

glial precursors?

Active source of

glial precursors?

Criscrossing

streams of

migrating

neurons

growing

axons ipsi-

laterally

and contra-

laterally

Crisscrossing

streams of

migrating

neurons

growing

axons ipsi-

laterally

and contra-

laterally

Dorsal

neuroepithelium

Dorsal

neuroepithelium

Intermediate

neuroepithelium

Intermediate

neuroepithelium

Ventral

neuroepithelium

Ventral

neuroepithelium

Sojourn zone

of premigratory

neurons destined

ffor the dorsal hornor the dorsal horn

Sojourn zone

of premigratory

neurons destined

for the dorsal horn

Dorsal

root

Migrating and settling

dorsal horn neurons

Sulcus

limitans

Dorsal funiculus

Diamond-shaped spinal canal

with expanding dorsal and

receding ventral components

Ventral white

commissure

Ventral white

commissure

D

o

r

s

a

l

fu

n

ic

u

lu

s

D

o

r

s

a

l

fu

n

ic

u

lu

s

L

a

t

e

r

a

l

f

u

n

i

c

u

l

u

s

L

a

t

e

r

a

l

f

u

n

i

c

u

l

u

s

V

e

n

tr

al

f un icu

lu

s

V

e

n

tr

al

f un icu

lu

s Incipient

ventral median

fissure

Large arrows extending from the neuroepithelium indicate many migrating neurons.
Arrows in the spinal canal indicate the direction of expansion or contraction.

PLATE 8B